Chlorine as a substituent – from quantum chemistry and photoelectron spectroscopy
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The role of chlorine as a substituent upon ionization, protonation and electrophilic addition of HCl has been studied by means of computational chemistry and photoelectron spectroscopy. Gas-phase carbon 1s photoelectron spectra of four chlorinated methanes (CClnH4−n, n=1,2,3,4), six chlorinated ethenes (C2ClnH6−n) and seven chlorinated propenes are recorded. The spectra are analyzed by means of theoretical modeling and ionization energies for each inequivalent carbon are extracted. Furthermore, ground-state potentials for each site are computed and thereby the ionization energies can be decomposed into contributions from the ground state and relaxation, e.g. delocalization of charge in the final state. It is found that chlorine primarily acts by making the ground-state potential of the neighboring carbon more positive. However, upon ionization, chlorine also donates a significant amount of electrons both to a neighboring carbon and to a second-nearest π-bonded neighbor.
Activation energies for the electrophilic addition of HCl to the chlorinated ethenes and propenes are computed and probably we overestimate the energies by about 10% compared to experimental results. Protonation enthalpies are predicted with uncertainties of 0.09 eV or less. We find that chlorine upon protonation act as an effective electron donor via the π-system if in a neighboring position to the protonated carbon and that the effective donation is much larger for protonation than for ionization and electrophilic addition. The explanation is that the protonated site has an enhanced ability to accept electrons compared to if the same site is ionized or is subject to an electrophilic addition.
At room temperature, both 3-chloropropene and 2,3-dichloropropene possess two stable rotational conformers. As a part of the theoretical modeling of a spectrum, we predict a theoretical vibrational lineshape for each chemical inequivalent site in the molecule. As the theoretical lineshapes for each of these rotamers are qualitatively different, the relative intensities of the vibrational lineshapes could be optimized and thereby relative populations could be determined.
Adsorption of 1,1-dichloroethene (Cl2C=CH2) to a Si(111)-7×7 surface is studied by means of XPS. It is found that 1/3 of the molecules break one C-Cl bond and 2/3 break both C-Cl bonds when chemisorbing to the surface. Physisorption spectra were compared to gas-phase spectra of the same compound, and it is found that except for broadening caused by the polarizable surface, the gas-phase spectrum constitutes an excellent model for the spectrum of the physisorbed species. We recommend that gas-phase spectra are used on a routine basis when assigning spectra of physisorbed species.
Comparing to experiments, we find that we are able to compute theoretical ionization energies within chemical accuracy, e.g. within 0.04 eV. However, our computed C-Cl bond lengths have deviations in the range of 0.6 pm whereas the desired accuracy is ±0.1 pm. The inaccuracy is related to a very slow basis set convergence for chlorine, making it a demanding substituent to model.